Silver-containing photothermographic imaging materials (that is, thermally developable photosensitive imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, X-radiation, or ultraviolet, visible, or infrared radiation) and developed by the use of thermal energy. These materials, also known as “dry silver” materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a “developer,” may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent. In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging layer(s).
Differences Between Photothermography and Photography
The imaging arts have, long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
In photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50° C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30° C. to 50° C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example a silver carboxylate or a silver benzotriazole) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself at least partially converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the “chemistry” for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. Even in so-called “instant photography,” the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of “fog” or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is in the aqueous fixing step).
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette's Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74–75, in Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94–103, and in M. R. V. Sahyun, J. Imaging Sci. Technol. 1998, 42, 23.
Problem to be Solved
Thermographic and photothermographic materials generally include a protective outer layer to protect the imaging layer and eventual image from damage from imaging equipment, spills, debris, and fingerprints.
Various overcoats or barrier layers have been developed to address these problems. For example, it is known from U.S. Pat. No. 5,422,234 (Bauer et al.) and U.S. Pat. No. 5,989,796 (Moon) to use a hydrophilic surface overcoat layer in photothermographic materials. Various barrier layer polymers are described in U.S. Pat. No. 6,352,819 (Kenney et al.), U.S. Pat. No. 6,250,561 (Miller et al.), U.S. Pat. No. 6,352,820 (Bauer et al.), and U.S. Pat. No. 6,420,102 (Bauer et al.).
Cellulose acetate polymers and their ester derivatives have been widely described as useful in overcoats of photothermographic materials. Other overcoat formulations are described in U.S. Pat. No. 5,536,696 (Uyttendaele et al.), U.S. Pat. No. 4,741,992 (Przezdziecki et al.), U.S. Pat. No. 5,759,752 (Uyttendaele et al.), and U.S. Pat. No. 6,344,313 (Goto et al.).
Many conventional thermally developable materials have layers formulated in various organic solvents, but there is interest for providing thermally developable imaging layers that are coated out of aqueous solvents to minimize the environmental impact of organic solvents. However, it is difficult to provide protective outer layers for such aqueous thermally developable imaging layers that are sufficiently resistant to water and damage from handling and imaging equipment. Moreover, it has been difficult to formulate protective layers that will inhibit the diffusion of imaging components from aqueous-based thermally developable imaging layers containing gelatin and other hydrophilic binders. The result of this chemical migration is reduced image density and photographic speed.
There remains a need for thermally developable materials comprising aqueous-based imaging layers that are protected by water-resistant and durable outermost protective layers that reduce or eliminate the diffusion of imaging components.